This disclosure relates generally to a self-levelling plastisol composition. More specifically, the plastisol composition described herein contains a polymeric resin such as PVC, a plasticizer, a wax and, optionally, a rheological additive, wherein the amount of wax exceeds the amount of rheological additive on a weight-by-weight basis. Methods for its use as a sealant in the automotive industry are also described.
Plastisols are widely used in many industrial processes as adhesives and sealers. See, e.g., U.S. Pat. Nos. 4,146,520; 4,151,317; 4,268,548; 4,440,900; 4,533,524; 4,673,710; 4,851,464; 4,900,771; 5,032,432; 5,039,768; 5,130,200, 5,143,650; 5,160,628; and 5,205,963; and European Patent App. No. 171,850. In general, adhesives are employed to bind various substrates together while sealants are employed to produce load bearing elastic joints between two or more surfaces and to prevent the passage of air, water and dirt therethrough. The automotive industry, in particular, is a major user of both the adhesives and sealants. Automobiles are assembled from several structural components which are joined together in various fashions depending on particular components and the degree of stress that will have to be endured. For certain assembly steps an adhesive composition is applied as a liquid and subsequently hardened to provide sufficient bonding strength. For example, adhesives are utilized in the assemblies of door panels, quarter panels, tailgates and roofs. These same assemblies will typically employ sealant compositions at a later stage in the assembly line. Still other automobile assemblies which are welded or bolted together use sealant compositions in their seams. The wheel house, shock tower, rocker panel, firewall, floor hem flange, floorplan, and trunk are a few examples of where sealants, but not adhesives, are employed.
Typically, automobile body assembly lines contain separate body shop and paint shop areas where adhesives and sealants are individually employed in each respective area. Adhesive compositions applied in the body shop area can be high strength epoxy or modified-epoxy adhesives which are capable of bonding to oily galvanized steel. In some cases, vinyl plastisols can be employed, however, these applications are normally limited to situations that do not require high bonding performance. In any case, these materials are applied at room temperature and later cured through exposure to heat. In order to apply these materials, they must be of low enough viscosity to be mechanically pumped with adequate flow rates. Normally, because of their low viscosity these materials are easily displaced when exposed to liquid impingement by various cleaning solutions (washes) to which the assembly body parts are exposed. Once the assembled parts are moved from the body shop area to the paint shop area, they are subjected to sealing, painting, and final oven curing. Sealant compositions have been found to have those needed characteristics.
Plastisols have been employed as sealants in the paint shop area of the body assembly line. The plastisols adhere well to primed metal surfaces and can be painted over. They are durable enough to withstand normal weather and user exposure. Another important quality of the plastisols is that they are not expensive. In general, plastisols, are dispersions of polymeric resins in plasticizers. Examples of such polymeric resins include poly(vinyl chloride), poly(vinyl acetate) and copolymers of vinyl chloride and vinyl acetate. Other polymers can be employed as well in the preparation of plastisols. Plasticizers are high boiling liquids which attack and plasticize the polymeric resin particles. The plastisols are liquids which are applied at room temperature to the substrate. The liquid is converted to a solid through exposure to heat. In effect, the heat causes the dispersed resin particles to fuse together or dissolve into the plasticizer. A solid product results upon subsequent cooling.
Waxes have been employed in plastisol compositions but typically in combination with rheological additives, e.g., thixotropic agents such as precipitated calcium carbonates, silicas and clay derivatives. Examples of such waxes include amorphous waxes, carnauba waxes, castor oil waxes and synthetic waxes. In general, the weight ratio of wax to rheological additive employed in plastisol compositions have ranged from about 1:2 to about 1:20.
One problem associated with the use of a plastisol having the wax to rheological additive ratio in the aforestated range is that during the application of the plastisol as a sealant in the paint shop area, it does not flow evenly, i.e., self-level, to provide a smooth finish. Referring to FIG. 1a, the plastisol is initially applied as a thick material 10 at room temperature to fill seam 30 of substrate 20 and/or body joints which must be sealed. As the temperature is increased, the plastisol exhibits limited flow characteristics due to the large amount of rheological additive as compared to the amount of wax. Thus, as depicted in FIG. 1b, when the applied plastisol 10 on substrate 20 is painted over, major imperfections such as tack, run-offs, bumps, ridges, etc. are visually apparent.